The present invention relates to a porous film and production thereof. More particularly, the invention relates to a porous film produced from a composition comprising a polyolefin and a filter and a process for producing the porous film, which film is suited for use as air-permeable sheet for medical use or other purposes.
Uniaxially stretching of films or sheets obtained by melt-forming a filler-blended polyolefin composition has been commonly practiced as a technique for producing porous films. However, porous films obtained in such conventional methods has problems in anisotropy of their properties, particularly relating to balance of tensile strength in the machine direction (stretching direction) and the transverse direction and in surface strength.
One method for improving anisotropy of film properties and surface strength is to form a porous film by stretching an unstretched film at as low a stretch ratio as possible. However, no satisfactory results have yet been obtained with this method.
Biaxial stretching of a unstretched film is also conceivable as a means for realizing said improvement, but this method involves problems of high equipment cost and strict control of operations because of the limited range of stretching conditions.
The present inventors have pursued the studies for eliminating these defects in the conventional porous films and providing a uniaxially stretched porous film which is well balanced in such properties as tensile strength and moisture preferably and also has high surface strength to enable greater reduction of thickness compared with conventional porous films, and as a result, the present inventors have accomplished the present invention.